Laser welding method

ABSTRACT

A laser welding method includes emitting two laser beams along a weld line from an upper surface side of a workpiece, the two laser beams being transmitted through different optical fibers and having in-focus spot diameters of 0.3 mm or larger; emitting the laser beams such that a leading laser beam of the two laser beams and a trailing laser beam of the two laser beams are each inclined toward a direction in which welding proceeds at an incident angle with respect to a direction perpendicular to an upper surface of the workpiece, the leading laser beam being ahead of the trailing laser beam on the upper surface of the workpiece in the direction in which welding proceeds, the trailing laser beam being behind the leading laser beam; and setting the incident angle of the leading laser beam to be larger than the incident angle of the trailing laser beam.

TECHNICAL FIELD

The present invention relates to a laser welding method with whichspatter formed during welding is prevented from adhering to an uppersurface of a workpiece or an optical component and undercut or underfill(that is, depression) is prevented from occurring on a back surface ofthe workpiece.

BACKGROUND ART

Laser welding can have high-energy density and can achieve a large depthof penetration and high speed welding. Thus, laser welding is expectedas a high-efficiency welding method. Since the laser welding causesmelting at extremely localized points, the effect of heat applied to aworkpiece (for example, a thin steel sheet, a thick steel plate, astainless steel plate, or a stainless steel sheet) is small anddistortion or deformation is slight, leading to a high-quality weldjoint. Thus, the laser welding has practically been used in the field ofthin steel sheets such as automobile thin steel sheets on an assemblyline of members of framework or automobile bodies. Also in the field ofthick steel plates, practical use of laser welding has seriously beenconsidered since the weldable plate (sheet) thickness is increased as aresult of current commercial sales of highly efficient laser weldingmachines that can perform optical fiber transmission at high power.

In the laser welding, however, a workpiece rapidly melts because ahigh-energy density laser beam is converged by an optical component andemitted to a weld portion. Thus, the molten metal may scatter around asspatter from the formed molten weld pool. If the scattered spatteradheres to the workpiece, the external appearance of the weld portion isspoiled. Furthermore, if spatter adheres to a guard glass or an opticalcomponent such as a lens, properties of a laser beam such as a focusingproperty or irradiance level are changed and the performance of thelaser welding becomes unstable.

If a large amount of spatter is generated, the amount of molten metal inthe molten weld pool decreases, thereby making welding defects such asundercut or underfill (i.e., depression) more likely to occur. Theoccurrence of undercut or underfill brings about reduction in strengthof the weld portion.

In view of these problems, Patent Literature 1, for example, discloses atechnology of preventing spatter from scattering into a laser workinghead by using a laser working head having a double-pipe nozzleconfiguration and forming a shielding curtain from assist gases ejectedfrom an external nozzle of the head.

Patent Literature 2 discloses a technology of preventing underfill andspatter from occurring by feeding a filler wire to a weld portionsubjected to laser welding while swinging the filler wire.

Patent Literature 3 discloses a technology of preventing spatter fromadhering to a laser working head and a workpiece by ejecting a fluidsideways to a space between a laser working head and a workpiece.

Patent Literature 4 discloses a technology of preventing spatter fromadhering to an optical component or a workpiece by blowing a gassideways from a position near the workpiece toward spatter scatteringfrom a molten weld pool formed by being irradiated with a laser beam.

CITATION LIST Patent Literature

PTL 1: Japanese Unexamined Patent Application Publication No. H11-123578

PTL 2: Japanese Unexamined Patent Application Publication No.2004-330299

PTL 3: Japanese Unexamined Patent Application Publication No.2003-334686

PTL 4: Japanese Unexamined Patent Application Publication No.2009-166050

SUMMARY OF INVENTION Technical Problem

Although the technology disclosed in Patent Literature 1 can preventspatter from adhering to the inside of a laser working head, thetechnology cannot prevent spatter from adhering to the tip of the laserworking head or a workpiece.

With the technology disclosed in Patent Literature 2, the components ofa filler wire to be used change the composition of the weld metal,thereby changing the properties of the weld metal. Thus, a filler wireappropriate for the components of the workpiece has to be selected.Consequently, the burden of stock management of filler wires orproduction control of filler wire selection is increased.

With the technology disclosed in Patent Literature 3, when a workpieceto be welded has a large plate thickness and requires a large laserpower, the amount of spatter scattering around increases. Thus, spattercannot be completely prevented from adhering to the laser working headand the workpiece.

The technology disclosed in Patent Literature 4 is effective forpreventing adhesion of spatter formed on the upper surface side of theworkpiece (that is, the side irradiated with a laser beam). With thetechnology, however, scattering spatter is removed by blowing, and thusthe amount of molten metal in the molten weld pool is reduced andwelding defects such as an undercut or an underfill are more likely tobe formed on the back surface of the workpiece.

In view of these circumstances, an object of the present invention is toprovide a laser welding method relating to a butt welding methodincluding welding by irradiating a groove formed between butt surfacesof workpieces with a laser beam from an upper surface side, the laserwelding method using no filler wires, preventing spatter from scatteringduring welding and from adhering to upper surfaces of the workpieces andan optical component, and preventing an undercut or an underfill frombeing formed on the back surface of the workpiece.

Solution to Problem

The inventors noticed that all the technologies disclosed in PatentLiteratures 1 to 4 are to perform laser welding by irradiating aworkpiece with a single laser beam emitted perpendicularly to theworkpiece. In the case where a workpiece is irradiated with aperpendicularly emitted single laser beam, the energy of the laser beamis focused on the portion of the workpiece irradiated with the laserbeam. Thus, the temperature of a molten metal significantly increasesand the molten metal swings. Consequently, not only spattering is morelikely to occur on the upper surface of the workpiece, but also anundercut or an underfill is more likely to be formed on the back surfaceof the workpiece.

On the other hand, when two laser beams are used and arranged along aweld line to divide the energy, an increase in temperature of the moltenmetal or swing of the molten metal can be prevented. Thus, the amount ofspatter generated on the upper surface of the workpiece can be reducedand an undercut or an underfill can be prevented from being formed onthe back surface of the workpiece. By emitting a laser beam (hereinafterreferred to as a leading laser beam) that is ahead of the other laserbeam (hereinafter referred to as a trailing laser beam) in a directionin which welding proceeds on the upper surface of the workpiece suchthat the leading laser beam and the trailing laser beam are inclinedtoward the direction in which welding proceeds, by setting the incidentangle of the leading laser beam to be larger than the incident angle ofthe trailing laser beam, and by emitting the leading laser beam and thetrailing laser beam such that the leading laser beam and the trailinglaser beam do not cross each other inside the workpiece, the laser beamsare not focused on a single point inside the workpiece, therebyincreasing effects of reducing the amount of spatter and preventingundercut or underfill from occurring. Although the details regarding themechanism that prevents spattering and undercut or underfill fromoccurring are not known, the following reasons have been presumed.Spatter is prevented from scattering by dividing the energy into twolaser beams emitted onto a steel sheet at incident angles, preheatingthe steel sheet using one of the laser beams that is ahead of the otherwhile suppressing spattering, and then melting the steel sheet using thetrailing laser beam. Here, the incident angle of each laser beamindicates the angle, formed, between a direction perpendicular to anupper surface of a workpiece and a direction in which the laser beam isemitted.

The present invention is made in view of these findings.

Specifically, the present invention provides a laser welding methodincluding: emitting two laser beams along a weld line from an uppersurface side of a workpiece, the two laser beams being transmittedthrough different optical fibers and having in-focus spot diameters of0.3 mm or larger; emitting the laser beams such that a leading laserbeam of the two laser beams and a trailing laser beam of the two laserbeams are each inclined toward a direction in which welding proceeds atan incident angle with respect to a direction perpendicular to an uppersurface of the workpiece, the leading laser beam being ahead of thetrailing laser beam on the upper surface of the workpiece in thedirection in which welding proceeds, the trailing laser beam beingbehind the leading laser beam on the upper surface of the workpiece inthe direction in which welding proceeds; and setting the incident angleof the leading laser beam to be larger than the incident angle of thetrailing laser beam.

In addition, the present invention is the laser welding method in whichlaser welding is performed by setting a gap between the center of anirradiated area of the upper surface of the workpiece irradiated withthe leading laser beam and the center of an irradiated area of the uppersurface of the workpiece irradiated with the trailing laser beam to be6×D_(max) or smaller where D_(max) is a larger spot diameter among aspot diameter Da of the leading laser beam and a spot diameter Db of thetrailing laser beam, and by setting a gap between the center of anemerging area of a back surface of the workpiece from which the leadinglaser beam emerges and the center of an emerging area of the backsurface of the workpiece from which the trailing laser beam emerges soas to fall within a range from 2×D_(max) to 12×D_(max) with respect tothe larger spot diameter D_(max).

Preferably, the incident angles of the leading laser beam and thetrailing laser beam fall within a range from 5 to 50°.

Advantageous Effects of Invention

According to the present invention, when butt welding is performed,spatter can be prevented from scattering from the upper surface of aworkpiece during welding and an undercut or an underfill can beprevented from being formed on the back surface of the workpiece.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a perspective view schematically illustrating a case to whichthe present invention is applied where welding is performed by using twolaser beams.

FIG. 2 is a plan view schematically illustrating an example of anarrangement of an irradiated area of the upper surface of a steel sheetillustrated in FIG. 1 irradiated with the leading laser beam, anirradiated area of the upper surface irradiated with the trailing laserbeam, and a weld line.

FIG. 3 is a side view schematically illustrating an example of anarrangement of the leading laser beam and the trailing laser beamillustrated in FIG. 1 and lines perpendicular to the upper surface of asteel sheet.

FIG. 4 is a perspective view schematically illustrating a second exampleof the present invention to which the present invention is applied wherewelding is performed by using two laser beams.

DESCRIPTION OF EMBODIMENTS

FIG. 1 is a perspective view schematically illustrating a case to whichthe present invention is applied where butt welding is performed onworkpieces using two laser beams. The arrow A in FIG. 1 indicates adirection in which welding proceeds. A weld line 2 is a line thatindicates a joint at which butted steel sheets 1 are joined. FIG. 1illustrates a deep cavity 4 (hereinafter referred to as a keyhole),which is formed as a result of emission of laser beams 3 a and 3 b, anda molten metal 5 formed around the keyhole 4 in perspective.Hereinbelow, a case is described where welding is performed onworkpieces 1 by taking steel sheets as examples of the workpieces 1.

In the present invention, two laser beams 3 a and 3 b are emitted froman upper surface side of steel sheets 1 along a weld line 2 between thesteel sheets 1. At this time, if a laser beam transmitted through asingle optical fiber is divided into two beams by an optical component(for example, a prism) and the two beams are emitted, incident angles ofthe two beams, which will be described below, cannot be individuallyset. Moreover, a gap between the center portions of irradiated areas,which will be described below, and the spot diameters cannot beindividually set. For this reason, the two laser beams 3 a and 3 b arerequired to be transmitted through different optical fibers.

One laser oscillator or two laser oscillators may be used. In the casewhere there is only one laser oscillator and two laser beams are to betransmitted, an oscillated laser beam is divided into two in theoscillator and then the two laser beams may be transmitted throughdifferent optical fibers.

(1) Gap Between Irradiated area Irradiated with Laser Beam and Weld Lineon Upper Surface of Steel Sheet

As illustrated in FIG. 1, the laser beams 3 a and 3 b are arranged atthe front and back along the weld line 2. One laser beam that is aheadof the other laser beam on the upper surface of the steel sheet 1 in thedirection in which welding proceeds is defined as a leading laser beam 3a, while the trailing laser beam that is behind the leading laser beam 3a is defined as a trailing laser beam 3 b. FIG. 2 is a plan viewschematically illustrating an example of an arrangement of an irradiatedarea 7 a of the upper surface of the steel sheet 1 irradiated with theleading laser beam 3 a, an irradiated area 7 b of the upper surfaceirradiated with the trailing laser beam 3 b, and the weld line 2.

Preferably, the centers of the irradiated areas 7 a and 7 b of the uppersurface of the steel sheet 1 respectively irradiated with the leadinglaser beam 3 a and the trailing laser beam 3 b are positioned so as tocoincide with the weld line 2, as illustrated in FIG. 2. It is, however,difficult to perform welding while these beams 3 a and 3 b are keptbeing positioned in the above-described manner, and thus the centers ofthe irradiated areas 7 a and 7 b of the upper surface of the steel sheet1 do not always coincide with the weld line 2 during the weldingoperation. When the gap between the weld line 2 and the center of eachirradiated area 7 a or 7 b increases, the leading laser beam 3 a or thetrailing laser beam 3 b deviates from the butt groove and thus weldingdefects such as incomplete melting of the groove are more likely tooccur.

Although the centers of the irradiated areas 7 a and 7 b do not coincidewith the weld line 2, if welding is performed such that the weld line 2passes the range within the irradiated areas 7 a and 7 b, weldingdefects do not occur. Thus, it is preferable that the gap between theweld line 2 and the center of each irradiated area 7 a or 7 b be withinthe radius of the irradiated area 7 a or 7 b.

(2) In-Focus Spot Diameters of Leading Laser Beam and Trailing LaserBeam

The in-focus spot diameters of the leading laser beam 3 a and thetrailing laser beam 3 b have to be 0.3 mm or larger. Here, the in-focusspot diameter is a beam diameter at a focused position when each laserbeam 3 a or 3 b is optically focused. Thus, the laser beam has thehighest energy density at the in-focus position. When the in-focus spotdiameters of the laser beams 3 a and 3 b are smaller than 0.3 mm, a weldbead 6 has a narrow width during welding and part of the groove remainsunmolten. On the other hand, if the spot diameters exceed 1.2 mm, thelaser beams have a small energy density and thus a keyhole 4 is morelikely to become unstable. Thus, preferably, the in-focus spot diametersof the laser beams 3 a and 3 b are 1.2 mm or smaller.

The keyhole 4 is formed due to evaporating pressure and evaporatingreaction force that occur when the steel sheet 1 melts and the moltenmetal 5 evaporates as a result of emission of the laser beams 3 a and 3b. Thus, the keyhole 4 needs to be stabilized in order to stably performbutt welding on the steel sheets 1 by applying the present invention.

Preferably, the in-focus spots of the laser beams 3 a and 3 b have acircular shape, but may have an oval figure. In the case where the spotshave an oval figure, the minor axis of the in-focus spot of each beam isset so as to exceed 0.3 mm or larger. In addition, the minor axis ispreferably 1.2 mm or smaller for the same reason as in the case of thecircular spot.

(3) Distance from Upper Surface of Steel Sheet to Focus

The distance from the upper surface of the steel sheet 1 to the focus ofeach laser beam 3 a or 3 b is denoted by t (mm) and the thickness of thesteel sheet 1 is denoted by T (mm). If the distance t from the uppersurface of the steel sheet 1 to the focus exceeds −3×T (specifically, 3Tupward from the upper surface), the position of the focus is too high,making it difficult to stably maintain the keyhole 4. On the other hand,if the distance t exceeds 3×T (specifically, 3T downward from the uppersurface), the position of the focus is too low, making spatter morelikely to be formed from the back surface of the steel sheet 1. Thus,the distance t from the upper surface of the steel sheet 1 to the focusis preferably set so as to fall within a range from −3×T to 3×T.

(4) Incident Angle of Leading Laser Beam and Incident Angle of TrailingLaser Beam

FIG. 3 is a side view schematically illustrating an example of anarrangement of the leading laser beam 3 a and the trailing laser beam 3b illustrated in FIG. 1 and lines perpendicular to the upper surface ofthe steel sheet 1. As illustrated in FIG. 3, the leading laser beam 3 aand the trailing laser beam 3 b are emitted onto the upper surface ofthe steel sheet 1 while being inclined toward the direction in whichwelding proceeds indicated by the arrow A. When an angle θa formedbetween the leading laser beam 3 a and a line perpendicular to the uppersurface of the steel sheet 1 is defined as an incident angle of theleading laser beam 3 a and an angle θb formed between the trailing laserbeam 3 b and a line perpendicular to the surface of the steel sheet 1 isdefined as an incident angle of the trailing laser beam 3 b, theincident angles are set so as to satisfy θa>θb.

In addition, the leading laser beam 3 a and the trailing laser beam 3 bare arranged so as not to cross each other inside the steel sheet 1.When the distance from the upper surface of the steel sheet 1 to anintersection of the leading laser beam 3 a and the trailing laser beam 3b is denoted by X (mm) and the sheet thickness of the steel sheet 1 isdenoted by T (mm), it is preferable that X be set within a range 0(i.e., the upper surface of the steel sheet 1)≦X≦2×T (i.e., 2T upwardfrom the upper surface of the steel sheet 1). The reason for thissetting is as follows. In the case where X<0, the leading laser beam 3 aand the trailing laser beam 3 b cross each other inside the steel sheet1. Thus, the keyholes 4 of the leading laser beam 3 a and the trailinglaser beam 3 b are combined with each other and form a large keyhole,thereby easily forming a large amount of spatter. On the other hand, inthe case where X>2×T, the molten metal 5 separates and thus the moltenstate of butted portion becomes unstable, thereby easily forming a largeamount of spatter.

When the incident angle θa of the leading laser beam 3 a and theincident angle θb of the trailing laser beam 3 b are set so as tosatisfy θa<θb, the distance from the upper surface to the back surfaceof the steel sheet 1 for which the trailing laser beam 3 b transmitsincreases. Thus, the energy of the trailing laser beam 3 b is attenuatedand the heating efficiency is reduced. Thus, the melting state of thebutted portion that is being molten by the trailing laser beam 3 bbecomes unstable although the preheating effect of the leading laserbeam 3 a on the butted portion is obtained.

If, on the other hand, the incident angles θa and θb are set so as tosatisfy θa=θb, the keyholes 4 of the leading laser beam 3 a and thetrailing laser beam 3 b become more likely to be combined with eachother and form a large keyhole. Thus, a large amount of spatter may beformed.

For this reason, the incident angles of the leading laser beam 3 a andthe trailing laser beam 3 b have to be set so as to satisfy θa>θb.Specifically, the inclination angle θa of the leading laser beam 3 a isset to be larger to prevent spattering from occurring when preheatingthe butted portion. The inclination angle θb of the trailing laser beam3 b is set to be smaller to increase heating efficiency when melting thebutted portion.

Consequently, spattering can be made less likely to occur and thus anundercut or an underfill can be prevented from being formed.

If the incident angle θa of the leading laser beam 3 a is below 5°, theincident angle θa is too small and the leading laser beam 3 a operatessimilarly as in the case where the leading laser beam 3 a is verticallyemitted. Thus, the leading laser beam 3 a cannot have the effect ofpreventing spattering from occurring. On the other hand, if the incidentangle θa exceeds 50°, the distance from the upper surface to the backsurface of the steel sheet 1 for which the leading laser beam 3 atransmits increases and the energy of the leading laser beam 3 a isattenuated. Thus, the leading laser beam 3 a cannot have a sufficientlylarge preheating effect. Therefore, it is preferable that the incidentangle θa of the leading laser beam 3 a be within a range from 5 to 50°.

Similarly, if the incident angle θb of the trailing laser beam 3 b isbelow 5°, the incident angle θb is too small and the trailing laser beam3 b operates similarly as in the case where the trailing laser beam 3 bis vertically emitted. Thus, the trailing laser beam 3 b cannot have theeffect of preventing spattering from occurring. On the other hand, ifthe incident angle θb exceeds 50°, the distance from the upper surfaceto the back surface of the steel sheet 1 for which the trailing laserbeam 3 b transmits increases and the energy of the trailing laser beam 3b is attenuated. Thus, the trailing laser beam 3 b cannot penetrate themolten metal to a sufficient depth. Therefore, it is preferable that theincident angle θb of the trailing laser beam 3 b be within a range from5 to 50°.

(5) Gap Between Center Points of Leading Laser Beam and Trailing LaserBeam on Upper Surface of Steel Sheet

When a gap L₁ between the centers of the irradiated areas 7 a and 7 b ofthe upper surface of the steel sheet 1 respectively irradiated with theleading laser beam 3 a and the trailing laser beam 3 b is too large, themolten metal 5 separates and spattering is more likely to occur. Thus,the gap L₁ between the centers of the irradiated areas 7 a and 7 b onthe upper surface of the steel sheet 1 is set to be 6×D_(max) or smallerwhere D_(max) denotes a larger spot diameter among the in-focus spotdiameters Da and Db of the laser beams 3 a and 3 b.

(6) Gap Between Centers of Emerging Areas of Back Surface of Steel Sheetfrom which Leading Laser Beam and Trailing Laser Beam Emerge

As illustrated in FIG. 4, an emerging area of the back surface of thesteel sheet 1 from which the leading laser beam 3 a emerges ispositioned ahead in the direction in which welding proceeds indicated bythe arrow A and an emerging area of the back surface of the steel sheet1 from which the trailing laser beam 3 b emerges is positioned behindthe emerging area of the leading laser beam 3 a. A gap L₂ between thecenters of the emerging areas is set so as to fall within a range from2×D_(max) to 12×D_(max), so that the trailing laser beam 3 b canpenetrate the molten metal to a sufficient depth and an underfill can beprevented from being formed on the back surface of the steel sheet 1.

(7) Oscillator of Laser Beam

Any of various forms of oscillators can be used as a laser beamoscillator used in the present invention. Preferable examples of theoscillator include a gas laser using a gas (for example, carbon dioxidegas, helium-neon, argon, nitrogen, and iodine) as a medium; a solidlaser using a solid (for example, YAG into which a rare earth element isdoped) as a medium; and a fiber laser using a fiber as a laser mediuminstead of bulk. Alternatively, a semiconductor laser may be used.

In the above described manner, according to the present invention, buttwelding can be performed while spatter is prevented from scattering fromthe upper surface of a workpiece during the welding operation or whilean undercut or an underfill is prevented from being formed on the backsurface of the workpiece.

The present invention can be applied not only to the butt welding ofworkpieces (for example, thin steel sheets, thick steel plates,stainless steel plates, or stainless steel sheets) but also toperforming welding for manufacturing a welded pipe by forming suchworkpieces into a cylinder.

Example 1

As illustrated in FIG. 1, when butt welding is to be performed onstainless steel sheets (SUS304, having a thickness of 5 mm) used as theworkpieces 1, laser beams oscillated by two laser oscillators are usedas the leading laser beam 3 a and the trailing laser beam 3 b andirradiated areas 7 a and 7 b are positioned on the weld line 2 asillustrated in FIG. 2. Welding conditions are made as illustrated inTable 1. The distance t (mm) from the upper surface of each stainlesssteel sheet 1 to the focus is fixed to ½T with respect to the sheetthickness T. The distance X (mm) from the upper surface of eachstainless steel sheet 1 to the intersection of the leading laser beam 3a and the trailing laser beam 3 b is fixed to ½T with respect to thesheet thickness T. The laser beam incident angles θa and θb shown inTable 1 indicate the angles illustrated in FIG. 3. Incident angleshaving negative values indicate that the laser beams are emitted whilebeing inclined toward a direction opposite to the direction A in whichwelding proceeds.

Examples of the invention shown in Table 1 (joint Nos. 2, 4, 5, 6, and9) are formed by making the in-focus spot diameters of the leading laserbeam 3 a and the trailing laser beam 3 b satisfy the range according tothe present invention, by emitting the leading laser beam 3 a and thetrailing laser beam 3 b such that the laser beams 3 a and 3 b areinclined toward the direction in which welding proceeds, and by settingthe incident angle θa of the leading laser beam 3 a to be larger thanthe incident angle θb of the trailing laser beam 3 b.

In comparative examples shown in Table 1, a joint No. 1 is an exampleobtained when the incident angle θb of the trailing laser beam 3 b isset to 0° (that is, by vertically emitting the trailing laser beam 3 b),a joint No. 3 is an example obtained when the in-focus spot diameter ofthe leading laser beam 3 a is out of the range according to the presentinvention, a joint No. 7 is an example obtained when the trailing laserbeam 3 b is emitted by being inclined toward a direction opposite to thedirection in which welding proceeds, and a joint No. 8 is an exampleobtained when the leading laser beam 3 a and the trailing laser beam 3 bare emitted by being inclined toward a direction opposite to thedirection in which welding proceeds.

After laser welding was performed in the above-described ways, the uppersurface of each stainless steel sheet 1 was visually inspected to checkwhether or not spatter has adhered thereto. In addition, a weld bead onthe back surface of each stainless steel sheet 1 was visually inspectedto check whether or not an undercut or an underfill has been formed. Theresults of the inspections are shown in Table 2.

As is clear from Table 2, neither adhesion of spatter nor occurrence ofundercut/underfill was perceived in the examples according to theinvention.

Among the comparative examples illustrated in Table 2, in the joint No.1, a large amount of spatter adhered to the stainless steel sheets 1 andunderfill occurred in the weld bead since the joint No. 1 was obtainedby setting the incident angle θb of the trailing laser beam 3 b to 0°.In the joint No. 3, spattering was not prevented and underfill occurredin the weld bead since the joint No. 3 was obtained by setting thein-focus spot diameter of the leading laser beam 3 a to a smalldiameter. In the joint No. 7, a large amount of spatter adhered to thestainless steel sheet 1 and underfill occurred in the weld bead sincethe joint No. 7 was obtained by inclining the trailing laser beam 3 btoward a direction opposite to the direction A in which weldingproceeds. In the joint No. 8, a large amount of spatter adhered to thestainless steel sheet 1 and underfill occurred in the weld bead sincethe joint No. 8 was obtained by inclining the leading laser beam 3 a andthe trailing laser beam 3 b toward a direction opposite to the directionin which welding proceeds.

Example 2

As illustrated in FIG. 4, when butt welding is to be performed onstainless steel sheets and plates (SUS304, having thicknesses of 5 mmand 10 mm) used as the workpieces 1, laser beams oscillated by two laseroscillators are used as the leading laser beam 3 a and the trailinglaser beam 3 b and irradiated areas 7 a and 7 b are positioned on theweld line 2 as illustrated in FIG. 2. Welding conditions are made asillustrated in Table 3. The distance t (mm) from the upper surface ofeach stainless steel plate (sheet) 1 to the focus is fixed to ½T withrespect to the plate (sheet) thickness T. The distance X (mm) from theupper surface of each stainless steel plate (sheet) 1 to theintersection of the leading laser beam 3 a and the trailing laser beam 3b is set to 0, ¼T, ½T, or T with respect to the sheet thickness T. Thelaser beam incident angles θa and θb shown in Table 3 indicate theangles illustrated in FIG. 3. Incident angles having negative valuesindicate that the laser beams are emitted while being inclined toward adirection opposite to the direction in which welding proceeds indicatedby the arrow A.

Examples of the invention shown in Table 3 (joint Nos. 2 to 4, 6, 7, and11) are formed by making the gap L₁ between the centers of theirradiated areas 7 a and 7 b of the upper surfaces of the stainlesssteel plates (sheets) 1 irradiated with the leading laser beam 3 a andthe trailing laser beam 3 b and the gap L₂ between the centers of theemerging areas of the back surfaces satisfy the ranges according to thepresent invention and by emitting the leading laser beam 3 a and thetrailing laser beam 3 b such that the leading laser beam 3 a and thetrailing laser beam 3 b are inclined toward the direction in whichwelding proceeds.

In comparative examples shown in Table 3, a joint No. 1 is an exampleobtained when the incident angle θb of the trailing laser beam 3 b isset to 0° (that is, by vertically emitting the trailing laser beam 3 b),joint Nos. 5, 8, and 10 are examples obtained when the gap L₂ betweenthe centers of the emerging areas of the back surfaces is out of therange according to the present invention, a joint No. 9 is an exampleobtained when the gap L₁ between the centers of the irradiated areas ofthe upper surfaces and the gap L₂ between the centers of the emergingareas of the back surfaces are out of the ranges according to thepresent invention, and a joint No. 12 is an example obtained when thelaser beams 3 a and 3 b are emitted by being inclined toward a directionopposite to the direction in which welding proceeds.

After laser welding was performed in the above-described ways, the uppersurface of each stainless steel plate (sheet) 1 was visually inspectedto check whether or not spatter has adhered thereto. In addition, a weldbead on the back surface of each stainless steel plate (sheet) 1 wasvisually inspected to check whether or not an undercut or an underfillhas been formed. The results of the inspections are shown in Table 4.

As is clear from Table 4, neither adhesion of spatter nor occurrence ofundercut/underfill was perceived in the examples according to theinvention.

Among the comparative examples illustrated in Table 4, in the joint No.1, a large amount of spatter adhered to the stainless steel plates(sheets) 1 and underfill occurred in the weld bead since the joint No. 1was obtained by setting the incident angle θb of the trailing laser beam3 b to 0°. In the joint No. 5, spatter adhered to the stainless steelplates (sheets) 1 and underfill occurred in the weld bead since thejoint No. 5 is obtained by setting the incident angle θa of the leadinglaser beam 3 a so as to be out of the range according to the presentinvention and thus the preheating effect is not sufficiently large. Inthe joint Nos. 8 and 10, underfill occurred on the back surfaces sincethe joint Nos. 8 and 10 are obtained by setting the gap L₂ between thecenters of the emerging areas of the back surfaces so as to be out ofthe range according to the present invention. In the joint No. 9, alarge amount of spatter adhered to the stainless steel plates (sheets) 1and underfill occurred in the weld bead since the joint No. 9 isobtained by setting the gap L₁ between the centers of the irradiatedareas of the upper surfaces and the gap L₂ between the centers of theemerging areas of the back surfaces so as to be out of the rangesaccording to the present invention. In the joint No. 12, a large amountof spatter adhered to the stainless steel plates (sheets) 1 andunderfill occurred in the weld bead since the joint No. 12 is obtainedby inclining the leading laser beam 3 a and the trailing laser beam 3 btoward a direction opposite to the direction in which welding proceeds.

INDUSTRIAL APPLICABILITY

The present invention is remarkably effective from the industrial pointof view because, when butt welding is performed, spatter can beprevented from scattering from the upper surface of a workpiece duringwelding and an undercut or an underfill can be prevented from beingformed on the back surface of the workpiece.

REFERENCE SIGNS LIST

-   -   1 workpiece    -   2 weld line    -   3 a leading laser beam    -   3 b trailing laser beam    -   4 keyhole    -   5 molten metal    -   6 weld bead    -   7 a irradiated area irradiated with leading laser beam    -   7 b irradiated area irradiated with trailing laser beam    -   θa incident angle of leading laser beam 3 a    -   θb incident angle of trailing laser beam 3 b    -   L1 distance between centers of irradiated areas of upper surface        of workpiece irradiated with leading laser beam and trailing        laser beam    -   L2 distance between centers of emerging areas of back surface of        workpiece from which leading laser beam and trailing laser beam        emerge    -   Da in-focus spot diameter of leading laser beam 3 a    -   Db in-focus spot diameter of trailing laser beam 3 b    -   D_(max) larger spot diameter between spot diameter Da of leading        laser beam and spot diameter Db of the trailing laser beam

TABLE 1 Welding conditions Distance between center of Laser In-focusspot Incident angle irradiated areas Welding Join Laser power diameterof laser beam and welded line speed No. beam (kW) (mm) (deg.) (mm)(m/min) Remarks 1 Leading 10 0.52 θa 25 0 10 Comparative Trailing 100.52 θb 0 0 example 2 Leading 10 0.52 θa 40 0 10 Example of Trailing 100.52 θb 15 0 invention 3 Leading 10 0.27 θa 45 0 10 Comparative Trailing10 0.52 θb 20 0 example 4 Leading 7.5 0.81 θa 45 0 7.5 Example ofTrailing 9 0.81 θb 20 0.5 invention 5 Leading 10 1.1 θa 50 0 10 Exampleof Trailing 10 0.52 θb 30 0 invention 6 Leading 8 1.1 θa 45 0.5 7.5Example of Trailing 7.5 0.52 θb 20 0 invention 7 Leading 8 0.32 θa 15 07.5 Comparative Trailing 9 0.52 θb −10 0 example 8 Leading 7.5 0.81 θa−10 0 7.5 Comparative Trailing 9 0.48 θb −35 0 example 9 Leading 10 0.52θa 35 0 12 Example of Trailing 10 0.32 θb 10 0 invention

TABLE 2 Join Whether or not spatter adheres Appearance of back No. toupper surface of workpiece surface of workpiece Remarks 1 Large amountof spatter adhered Underfill occurred Comparative example 2 Not adheredGood Example of invention 3 Small amount of spatter adhered Underfilloccurred Comparative example 4 Not adhered Good Example of invention 5Not adhered Good Example of invention 6 Not adhered Good Example ofinvention 7 Large amount of spatter adhered Underfill occurredComparative example 8 Large amount of spatter adhered Underfill occurredComparative example 9 Not adhered Good Example of invention

TABLE 3 Welding conditions Distance L1 Distance L2 between centersbetween centers Distance X from upper Plate In-focus of irradiated ofemerging surface to intersection (Sheet) Laser spot Incident areas onupper Welding areas on back of leading laser beam Join thickness Laserpower diameter angle surface speed surface and trailing laser beam No. T(mm) beam (kw) (mm) (°) (mm) (m/min) (mm) (mm) Remarks 1 5 Leading 10 Da0.52 θa 30 1.4 10 4.3 1/2T Comparative Trailing 10 Db 0.52 θb 0 example2 5 Leading 10 Da 0.52 θa 35 1.1 10 3.2 1/2T Example of Trailing 10 Db0.52 θb 15 invention 3 5 Leading 10 Da 0.52 θa 40 2.4 10 4.0 T Exampleof Trailing 10 Db 0.52 θb 20 invention 4 5 Leading 10 Da 0.48 θa 45 0.710 3.3 1/4T Example of Trailing 10 Db 0.52 θb 25 invention 5 5 Leading10 Da 0.52 θa 60 2.9 10 8.7 1/2T Comparative Trailing 10 Db 0.52 θb 30example 6 5 Leading 7 Da 0.48 θa 30 1 7.5 3 1/2T Example of Trailing 9Db 0.48 θb 10 invention 7 5 Leading 8 Da 0.52 θa 50 0 7.5 3.1 0 Exampleof Trailing 8 Db 0.52 θb 30 invention 8 5 Leading 8 Da 0.48 θa 45 2.17.5 6.2 1/4T Comparative Trailing 10 Db 0.48 θb 15 example 9 10 Leading10 Da 0.52 θa 45 3.2 4.5 9.5 1/2T Comparative Trailing 10 Db 0.48 θb 20example 10 10 Leading 10 Da 0.52 θa 50 1.5 5 7.7 1/4T ComparativeTrailing 10 Db 0.52 θb 30 example 11 10 Leading 10 Da 0.81 θa 45 2.7 6 81/2T Example of Trailing 10 Db 0.52 θb 25 invention 12 10 Leading 10 Da0.52 θa −5 4.9 5 9.8 T Comparative Trailing 10 Db 0.52 θb −30 example

TABLE 4 Whether or not spatter adheres Appearance of back Join No. toupper surface of workpiece surface of workpiece Remarks 1 Large amountof spatter Underfill occurred Comparative example adhered 2 Not adheredGood Example of invention 3 Not adhered Good Example of invention 4 Notadhered Good Example of invention 5 Large amount of spatter Underfilloccurred Comparative example adhered 6 Not adhered Good Example ofinvention 7 Not adhered Good Example of invention 8 Small amount ofspatter Underfill occurred Comparative example adhered 9 Large amount ofspatter Underfill occurred Comparative example adhered 10 Small amountof spatter Underfill occurred Comparative example adhered 11 Not adheredGood Example of invention 12 Large amount of spatter Underfill occurredComparative example adhered

The invention claimed is:
 1. A laser welding method comprising: emittingtwo laser beams along a weld line from an upper surface side of aworkpiece, the two laser beams being transmitted through differentoptical fibers and having in-focus spot diameters of 0.3 mm or larger;emitting the laser beams such that a leading laser beam of the two laserbeams and a trailing laser beam of the two laser beams are each inclinedtoward a direction in which welding proceeds at an incident angle withrespect to a direction perpendicular to an upper surface of theworkpiece, the leading laser beam being ahead of the trailing laser beamon the upper surface of the workpiece in the direction in which weldingproceeds, the trailing laser beam being behind the leading laser beam onthe upper surface of the workpiece in the direction in which weldingproceeds; and setting the incident angle of the leading laser beam withrespect to the perpendicular to be larger than the incident angle of thetrailing laser beam.
 2. The laser welding method according to claim 1,wherein laser welding is performed by setting a gap between the centerof an irradiated area of the upper surface of the workpiece irradiatedwith the leading laser beam and the center of an irradiated area of theupper surface of the workpiece irradiated with the trailing laser beamto be 6×D_(max) or smaller, where D_(max) is a larger spot diameteramong a spot diameter Da of the leading laser beam and a spot diameterDb of the trailing laser beam, and by setting a gap between the centerof an emerging area of a back surface of the workpiece from which theleading laser beam emerges and the center of an emerging area of theback surface of the workpiece from which the trailing laser beam emergesso as to fall within a range from 2×D_(max) to 12×D_(max) with respectto the larger spot diameter D_(max).
 3. The laser welding methodaccording to claim 1, wherein the incident angles of the leading laserbeam and the trailing laser beam fall within a range from 5 to 50°. 4.The laser welding method according to claim 2, wherein the incidentangles of the leading laser beam and the trailing laser beam fall withina range from 5 to 50°.